Ok, I have a question about swapping used carriers, bolts and uppers.  Let's pretend for the sake of this conversation that headspace is checked and turns out just fine.

I've heard 3 different things regarding this:

1.  Do not swap carriers/bolts in different uppers after they have been used/fired. (few people)

2.  It is ok to swap carriers/bolts in different uppers after they have been used/fired as long as they have less than 500 rounds of wear on them.  (bushmaster I think, a few others)

3.  It's fine to swap carriers and bolts in different uppers no matter how many rounds have been put through them as long as the carriers/bolts/uppers have no cracks or problems. (few people)

Sooooo.. which is it?

In general, it makes no difference about the carrier, unless it is worn out of spec.

The issue is the bolt. The locking lugs on the bolt wear to a specific upper.
Swapping the bolt, even if it passes headspace could in a blue moon not lock properly.
I would assume it would have to be badly worn for this to happen.

In general - keep your bolts with your upper.
Is a $40 bolt worth a failure that could cause injury?

Just my opinion.

~ s0ulzer0

So the bolt lugs wear, not the lugs in the barrel extention?  I guess what I'm asking is.. what about a brand new bolt in an OLD upper/barrel?

They both wear (barrel ext & bolt) but bolt more so. Putting a new bolt in an older upper
and it passes headspace should be just fine.
If the upper is really old/well used, check for excessive wear on the lugs of the barrel extention.

~ s0ulzer0

Well---let me give you a bit of personal experience. Last year I bought 5 used GI bolt assemblies. Based on finish wear, they all were out of early M16A1s, and one of them was so old it still had the white extractor spring insert, that hasn't been used since about 1968. It also showed the most finish wear. I tried, and headspaced, every one of them in four different AR barrels, Two Colt's, one old Bushmaster, and one almst new Rock River. One of the Colt's was my SP1, which I've owned since 1977, and which has had thousands of rounds thru it. The other Colt was a very late production M16A1 GI surp piece, which may have had a couple of thousand rounds thru it before it was pulled, and an A2 barrel/upper put on the lower.

Four of the five would not close on the No go guage of ANY of the four, and the fifth (the one w/the white insert), while it closed on the no go guage on the SP1 and Bushie barrals, would not close on the field guage.

Can't speak for aftermarket bolts, but when it comes to the Colt pieces, I don't think you can wear 'em out; which is why all my ARs have Colt bolts, and all but one have Colt carriers[:D]